Yes, the element is behind the box. You can get to it from there, but it requires some stretching and grunting, I've done it a few times. Removing the entire heater to do it is a bit of work and would be easy to damage other stuff since the rig is so old. Old Biscuit it exactly correct on what he says. BTW, the elements are readily available at your local home repair store. Usually about $12.
Regarding the brass plug. OB is correct, but I have a brass plug in mine and have no issues as I drain and flush my unit about 3 to 4 times a year as a FTer. I use the super-duper blue plumbers tape (Teflon with pipe dope between the layers of tape) on the threads. The brass and aluminum galvanic process doesn't have enough time to occur. Just sayin'. Leaving the plug in for an extended period may be a problem. Never use a cast iron plug.